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ABSTRACT: Tethyan Evolution of Central Asia

A. M. C. Sengor

The study area extends from the eastern shores of the Caspian Sea in the west to the Helan Shan and Longmen Shan in the east and from about 40°N parallel in the north to the neo-Tethyan sutures in the south, thus including what is called Middle Asia in the Soviet literature. In the region thus delineated lies the boundary between the largely late Paleozoic core of Asia (Altaids) and the Tethyside superorogenic complex. This boundary passes through continental objects that collided with nuclear Asia in the late Paleozoic to terminate its Altaid evolution. Subduction to the south of some of these had commenced before they collided (e.g., Tarim in the Kuen-Lun), in others later (e.g., South Ghissar area west of Pamirs). This subduction led, in the late Paleozoic, to the opening of marginal basins, at least one of which may be partly extant (Tarim). Giant subduction accretion complexes of Paleozoic to earliest Triassic age dominate farther south in the basement of Turan (mainly in Turkmenian SSR) and in the Kuen-Lun/Nan Shan ranges. No discrete continental collisions or any continental basement in these regions could be unequivocally recognized contrary to most current interpretations. Magmatic arcs that developed along the southern margin of Asia in the late Paleozoic to early Mesozoic grew atop these subduction-accretion complexes and record a gradual southerly migration of magmatism through time.

Subduction also dominated the northern margin of Gondwanaland between Iran and China in late Paleozoic time, although the record in Afghanistan and northwest Tibet is scrappy. It led to back-arc basin formation, which in Iran and Oman became neo-Tethys and, in at least parts of central Asia, the Waser-Mushan-Pshart/Banggong Co-Nu Jiang ocean. This ocean was probably connected with the Omani part of the neo-Tethys via the Sistan region. South-dipping subduction eliminated the Waser/Rushan-Pshart/Banggong Co-Nu Jiang ocean in the Late Jurassic to Early Cretaceous, partly triggering the onset of neo-Tethyan subduction south of the Helmand/South Pamir/Lhasa fragment, thus carrying orogenic activity outside central Asia. Most Alpide tectonism in central Asia that followed the collisional o literation of neo-Tethys was germanotype in character and created the giant rejuvenated intracontinental pop-up mountains such as Tien Shan and Kuen Lun and the intermontane ramp basins of the Tarim. These basins are the main hydrocarbon reservoirs in the region.

In summary, central Asia is a mosaic of accreted continental blocks and subduction-accretion complexes. The latter played a much greater role in the evolution of central Asia than hitherto supposed and formed the germanotype pop-up ridges in the neotectonic evolution.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990